This bug is a trivial one, but I wonder has anyone noticed, that when you touch on a book on the main page to open it, sometimes, the feedback (the border of the book image going bold) does not occur? Sometimes this leads to me touching it twice because I think it hasn't registered the touch (and that second touch becomes a page turn when the book opens).

It's been doing this for the previous few versions of software - and still there in 2.1.4 that I am running now.

Yes it happens enough to be noticable. I've never been able to pin down the exact circumstances that cause or cure it. Kobo is aware of it, but as you said in the grand scheme it's pretty minor so I'm not sure how much time (if any) is being spent looking into it.